The polyphenylene ether (oxide) resins are high performance engineering thermoplastics characterized by relatively high melt viscosities and softening points. The resins are useful in many commercial applications requiring high temperatures resistance, and can be admixed with other polymers, such as polystyrene resins, to form blends which are extrudable or moldable into articles of various shapes and sizes. Polyphenylene ether resins and methods of their preparation are described in U.S. Pat. Nos. 3,306,874 and 3,306,875 (Hay), in U.S. Pat. Nos. 3,257,357 and 3,257,358 (Stamatoff), and elsewhere. Blends of polyphenylene ether resin and various styrenic polymers are disclosed by Cizek in U.S. Pat. No. 3,383,435.
The polyphenylene ether resins are prone to undergoing discoloration, that is, yellowing, during processing at elevate temperatures and, after molding, upon exposure to ultraviolet (UV) light for extended periods. To improve the color stability of these resins or of blends made of the resins, stabilizers are sometimes added, a number of which are disclosed in the patent literature. For instance, Bennett in U.S. Pat. No. 3,563,934 teaches that an alkanolamine or a morpholine can be incorporated in a polyphenylene oxide resin to improve the flow and stability of the polymer during molding, and to enable lighter colored objects to be molded from the polymer.
Murayama, et al. in U.S. Pat. No. 3,640,928 disclose that various synthetic polymers can be stabilized against photo- and thermal deterioration by incorporating certain sterically hindered piperidines.
The use of hindered phenols as thermal stabilizers for compositions of polyphenylene ethers (oxides) is described in U.S. Pat. No. 3,700,750 (Yamanouchi, et al.) and U.S. Pat. No. 4,184,999 (Olander).
Because polyphenylene ether resin blends are now used to produce a wider range of commercial articles, many of which are exposed to sunlight or bright indoor light during regular use, there is a desirability to find additional ways in which the UV-stability of the polymer can be improved without detracting from other important physical properties.
In one previous development, it has been found that certain hindered amine compounds, which more specifically are hydrid forms of a hindered piperidine and a hindered phenol, are effective to stabilize polyphenylene ether resins and blends against UV light radiation effects. These compounds, which are described in U.S. Pat. No. 4,198,334 (Rasberger), apparently function as UV screening agents which absorb harmful light rays.
In a separate development, it has previously been found that certain compounds based on benzotriazole are also effective as UV light stabilizers for polyphenylene ether resins and blends. These compounds apparently act as antioxidants which block or interrupt the free radical photooxidation process in the polymer.
These two types are not without disadvantages, however. Both are susceptible to photooxidation and either undergo degradation or are consumed in the process of protecting the polymer. Thus, to compensate, the polymer must usually be loaded up with relatively larger amounts of these additives, which increases the cost of manufacture and can sometimes lead to sacrifices in other physical properties.